A well known low power RF receiver circuit comprises a low noise preamplifier and double-balanced mixer, using novel monolithic microstrip inductors and transformers for RF IC (integrated circuit) applications using submicron bipolar CMOS process technology, with particular application for personal communications systems.
For wireless personal communications systems, successful integration of high performance low power RF receivers, often referred to as RF "front ends", comprising a low noise preamplifier and mixer, depends heavily on the development of appropriate on-chip resonant circuits.
High frequency amplifiers in silicon integrated technology have traditionally been designed as broadband circuits using resistors (R) and capacitors (C) as the sole passive circuit elements. Typically, RC circuits are limited in their frequency range. Narrowband LC (inductor and capacitor) resonant circuits, based on GaAs technology offer higher frequency operation and reduced chip size, more suitable for modern high frequency communications systems, which are narrowband in nature and low noise.
Common LC circuit topologies have migrated progressively from discrete elements, to hybrids, to GaAs MMICs (monolithic microwave ICs). MMIC narrowband LC circuits designs are based on power matching of circuit elements. Gain is possible, ideally, up to almost fmax of the transistor. GaAs technologies have a clear advantage over silicon for this type of design, because the substrate is semi-insulating, and process options include airbridge interconnect schemes which facilitate fabrication of inductive elements.
Indeed, monolithic integration of microstrip components as inductive elements on silicon substrates has previously been considered impractical due to the high dielectric losses in the silicon substrate. However, development of multilevel metallization in modern silicon VLSI technologies has reduced the metal to substrate capacitance, thereby reducing dielectric losses at lower frequencies. Thus, more recently, interest has been revived in exploring the possibilities offered by integration of microstrip elements on silicon for cost reduced RF IC designs.
For example, U.S. Pat. No. 5,379,457 granted to N. M. Nguyen on Jan. 3, 1995 discloses a low noise active mixer. The mixer is based on a conventional Gilbert cell active mixer having two matched pairs of transistors which receive a local oscillator input signal, coupled to another matched pair of transistors which receive a radio frequency signal. The circuit generates an intermodulated output signal at the collectors of the local oscillator matched pairs. Noise degradation is reduced by replacing the standard radio frequency emitter degeneration resistor with a reactive element, thereby reducing thermal noise. Narrowband input matching is achieved by a series inductive element, e.g., a spiral inductor and an optional parallel capacitive element in line with the radio frequency input.